Quantitative analysis of microstructure evolution, stress partitioning and thermodynamics in the dynamic transformation of Fe-14Ni alloy

Dynamic transformation (DT) of austenite (γ) to ferrite (α) in the hot deformation of various carbon steels was widely investigated. However, the nature of DT remains unclear due to the lack of quantitative analysis of stress partitioning between two phases and the uncertainty of local distribution of substitutional elements at the interface in multi-component carbon steels used in the previous studies. Therefore, in the present study, a binary Fe-Ni alloy with α + γ duplex microstructure in equilibrium was prepared and isothermally compressed in α + γ two-phase region to achieve a quantitative analysis of microstructure evolution, stress partitioning, and thermodynamics during DT. γ to α DT during isothermal compression and α to γ reverse transformation on isothermal annealing under unloaded condition after deformation were accompanied by Ni partitioning. The lattice strains during thermomechanical processing were obtained via in-situ neutron diffraction measurement, based on which the stress partitioning behavior between γ and α was discussed by using the generalized Hooke's law. A thermodynamic framework for the isothermal deformation in solids was established based on the basic laws of thermodynamics, and it was shown that the total Helmholtz free energy change in the deformable material during the isothermal process should be smaller than the work done to the deformable material. Under the present thermodynamic framework, the microstructure evolution in the isothermal compression of Fe-14Ni alloy was well explained by considering the changes in chemical free energy, plastic and elastic energies, and the work done to the material. In addition, the stabilization of the soft α phase in Fe-14Ni alloy by deformation was rationalized since the γ to α transformation decreased the total Helmholtz free energy by decreasing the elastic and dislocation energies.